Mri-safe implantable medical device

ABSTRACT

A medical lead is provided for use in a pulse stimulation system of the type which includes a pulse generator for producing electrical stimulation therapy. The lead comprises an elongate insulating body and at least one electrical conductor within the insulating body. The conductor has a proximal end configured to be electrically coupled to the pulse generator and has a DC resistance in the range of 375-2000 ohms. At least one distal electrode is coupled to the conductor.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/954,563 filed on Nov. 24, 2010, and entitled MRI-SafeImplantable Medical Device, which is incorporated by reference hereinand which is a continuation of U.S. patent application Ser. No.10/945,739, filed on Sep. 20, 2004, and entitled MRI-Safe ImplantableMedical Device, which is incorporated by reference herein and whichclaims the benefit of U.S. Provisional Application No. 60/557,991 filedMar. 30, 2004, which is also incorporated by reference herein.

FIELD OF THE INVENTION

The present invention generally relates to implantable medical devices,and more particularly to an implantable medical device such as aneurostimulation system which, when used in an MRI (Magnetic ResonanceImaging) environment does not result in the generation of unwanted heat.

BACKGROUND OF THE INVENTION

Implantable medical devices are commonly used today to treat patientssuffering from various ailments. Such implantable devices may beutilized to treat conditions such as pain, incontinence, sleepdisorders, and movement disorders such as Parkinson's disease andepilepsy. Such therapies also appear promising in the treatment of avariety of psychological, emotional, and other physiological conditions.

One known type of implantable medical device, a neurostimulator,delivers mild electrical impulses to neural tissue using an electricallead. For example, to treat pain, electrical impulses may be directed tospecific sites. Such neurostimulation may result in effective painrelief and a reduction in the use of pain medications and/or repeatsurgeries.

Typically, such devices are totally implantable and may be controlled bya physician or a patient through the use of an external programmer.Current systems generally include a non-rechargeable primary cellneurostimulator, a lead extension, and a stimulation lead, and the twomain classes of systems may be referred to as: (1) Spinal CordStimulation (SCS) and (2) Deep Brain Stimulation (DBS).

An SCS stimulator may be implanted in the abdomen, upper buttock, orpectoral region of a patient and may include at least one extensionrunning from the neurostimulator to the lead or leads which are placedsomewhere along the spinal cord. Each of the leads (to be discussed indetail hereinbelow) currently contain from one to eight electrodes. Eachextension (likewise to be discussed in detail below) is plugged into orconnected to the neurostimulator at a proximal end thereof and iscoupled to and interfaces with the lead or leads at a distal end of theextension or extensions.

The implanted neurostimulation system is configured to send mildelectrical pulses to the spinal cord. These electrical pulses aredelivered through the lead or leads to regions near the spinal cord orthe nerve selected for stimulation. Each lead includes a small insulatedwire coupled to an electrode at the distal end thereof through which theelectrical stimulation is delivered. Typically, the lead also comprisesa corresponding number of internal wires to provide separate electricalconnection to each electrode such that each electrode may be selectivelyused to provide stimulation. Connection of the lead to an extension maybe accomplished by means of a connector block including, for example, aseries or combination of set-screws, ball-seals, etc. The leads areinserted into metal set screw blocks, and metal set screws aremanipulated to press the contacts against the blocks to clamp them inplace and provide an electrical connection between the lead wires andthe blocks. Such an arrangement is shown in U.S. Pat. No. 5,458,629issued Oct. 17, 1995 and entitled “Implantable Lead Ring Electrode andMethod of Making”.

A DBS system comprises similar components (i.e. a neurostimulator, atleast one extension, and at least one stimulation lead) and may beutilized to provide a variety of different types of electricalstimulation to reduce the occurrence or effects of Parkinson's disease,epileptic seizures, or other undesirable neurological events. In thiscase, the neurostimulator may be implanted into the pectoral region ofthe patient. The extension or extensions may extend up through thepatient's neck, and the leads/electrodes are implanted in the brain. Theleads may interface with the extension just above the ear on both sidesof the patient. The distal end of the lead may contain from four toeight electrodes and, as was the case previously, the proximal end ofthe lead may be connected to the distal end of the extension and held inplace by set screws. The proximal portion of the extension plugs intothe connector block of the neurostimulator.

Magnetic resonance imaging (MRI) is a relatively new and efficienttechnique that may be used in the diagnosis of many neurologicaldisorders. It is an anatomical imaging tool which utilizes non-ionizingradiation (i.e. no x-rays or gamma rays) and provides a non-invasivemethod for the examination of internal structure and function. Forexample, MRI permits the study of the overall function of the heart inthree dimensions significantly better than any other imaging method.Furthermore, imaging with tagging permits the non-invasive study ofregional ventricular function.

MRI scanning is widely used in the diagnosis of diseases and injuries tothe head. In fact, the MRI is now considered by many to be the preferredstandard of care, and failure to prescribe MRI scanning can beconsidered questionable. For example, approximately sixteen million MRIswere performed in 1996 followed by approximately twenty million in theyear 2000. It is projected that forty million MRIs will be performed in2004.

In an MRI scanner, a magnet creates a strong magnetic field which alignsthe protons of hydrogen atoms in the body and then exposes them to radiofrequency (RF) energy from a transmitter portion of the scanner. Thisspins the various protons, and they produce a faint signal that isdetected by a receiver portion of the scanner. A computer renders thesesignals into an image. During this process, three electromagnetic fieldsare produced; i.e. (1) a static magnetic field, (2) a gradient magneticfield, and (3) a radio frequency (RF) magnetic field. The main or staticmagnetic field may typically vary between 0.2 and 3.0 Tesla. A nominalvalue of 1.5 Tesla is approximately equal to 15,000 Gauss which is30,000 times greater than the Earth's magnetic field of approximately0.5 Gauss. The time varying or gradient magnetic field may have amaximum strength of approximately 40 milli-Tesla/meter at a frequency of0-5 KHz. The RF may, for example, produce thousands of watts atfrequencies of between 8-128 MHz. For example, up to 20,000 watts may beproduced at 64 MHz and a static magnetic field of 1.5 Tesla; that is, 20times more power than a typical toaster. Thus, questions have arisenregarding the potential risk associated with undesirable interactionbetween the MRI environment and the above-described neurostimulationsystems; e.g. forces and torque on the implantable device within the MRIscanner caused by the static magnetic field, RF-induced heating, inducedcurrents due to gradient magnetic fields, device damage, and imagedistortion. Of these interactions, the problems associated with inducedRF currents in the leads are most deserving of attention since it hasbeen found that the temperature in the leads can rise by as much as 25°Centigrade or higher in an MRI environment.

Accordingly, it would be desirable to provide an implantable medicaldevice that may be safely operated in an MRI environment. It would befurther desirable to provide an implantable medical device such as a SCSor DBS neurostimulation system that may be operated in an MRIenvironment without the generation of significant heat in the leads dueto induced RF currents. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description of the invention and the appendedclaims, taken in conjunction with the accompanying drawings and thisbackground of the invention.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a pulsestimulation system, comprising a pulse generator for producingelectrical stimulation and a conductive stimulation lead having aproximal end electrically coupled to the pulse generator, thestimulation lead having a DC resistance in the range of 375-2000 ohms.At least one distal electrode is provided on the distal end.Alternatively, high absolute impedance leads utilizing a combination ofDC resistance and impedance due to inductance are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe accompanying drawing, wherein like reference numerals denote likeelements; and

FIG. 1 illustrates a typical spinal cord stimulation system implanted ina patient;

FIG. 2 illustrates a typical deep brain stimulation system implanted ina patient;

FIG. 3 is an isometric view of the distal end of the lead shown in FIG.2;

FIG. 4 is an isometric view of the distal end of the extension shown inFIG. 2;

FIG. 5 is an isometric view of an example of a connector screw blocksuitable for connecting the lead of FIG. 3 to the extension shown inFIG. 4;

FIG. 6 is a top view of the lead shown in FIG. 2;

FIGS. 7 and 8 are cross-sectional views taken along lines 7-7 and 8-8,respectively, in FIG. 6;

FIG. 9 is a top view of an alternate lead configuration;

FIGS. 10 and 11 are longitudinal and radial cross-sectional views,respectively, of a helically wound lead of the type shown in FIG. 6;

FIGS. 12 and 13 are longitudinal and radial cross-sectional views,respectively, of a cabled lead;

FIG. 14 is an exploded view of a neurostimulation system;

FIG. 15 is a cross-sectional view of the extension shown in FIG. 14taken along line 15-15;

FIG. 16 illustrates a discrete inductor in the distal electrode of alead;

FIG. 17 is a cross-sectional view of a prismatic discrete inductor in adistal electrode; and

FIG. 18 is a cross-sectional view of a quadripolar coaxially-wound lead.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

FIG. 1 illustrates a typical SCS system implanted in a patient. As canbe seen, the system comprises a pulse generator such as a SCSneurostimulator 20, a lead extension 22 having a proximal end coupled toneurostimulator 20 as will be more fully described below, and a lead 24having a proximal end coupled to the distal end of extension 22 andhaving a distal end coupled to one or more electrodes 26.Neurostimulator 20 is typically placed in the abdomen of a patient 28,and lead 24 is placed somewhere along spinal cord 30. As statedpreviously, neurostimulator 20 may have one or two leads each havingfour to eight electrodes. Such a system may also include a physicianprogrammer and a patient programmer (not shown). Neurostimulator 20 maybe considered to be an implantable pulse generator of the type availablefrom Medtronic, Inc. and capable of generating multiple pulses occurringeither simultaneously or one pulse shifting in time with respect to theother, and having independently varying amplitudes and pulse widths.Neurostimulator 20 contains a power source and the electronics forsending precise, electrical pulses to the spinal cord to provide thedesired treatment therapy. While neurostimulator 20 typically provideselectrical stimulation by way of pulses, other forms of stimulation maybe used such as continuous electrical stimulation.

Lead 24 is a small medical wire having special insulation thereon andincludes one or more insulated electrical conductors each coupled attheir proximal end to a connector and to contacts 26 at its distal end.Some leads are designed to be inserted into a patient percutaneously(e.g. the Model 3487A Pisces—Quad® lead available from Medtronic, Inc.),and some are designed to be surgically implanted (e.g. Model 3998Specify® lead, also available from Medtronic, Inc.). Lead 24 may containa paddle at its distant end for housing electrodes 26; e.g. a Medtronicpaddle having model number 3587A. Alternatively, electrodes 26 maycomprise one or more ring contacts at the distal end of lead 24 as willbe more fully described below.

While lead 24 is shown as being implanted in position to stimulate aspecific site in spinal cord 30, it could also be positioned along theperipheral nerve or adjacent neural tissue ganglia or may be positionedto stimulate muscle tissue. Furthermore, electrodes/contacts 26 may beepidural, intrathecal or placed into spinal cord 30 itself. Effectivespinal cord stimulation may be achieved by any of these lead placements.While the lead connector at proximal end of lead 24 may be coupleddirectly to neurostimulator 20, the lead connector is typically coupledto lead extension 22 as is shown in FIG. 1. An example of a leadextension is Model 7495 available from Medtronic, Inc.

A physician's programmer (not shown) utilizes telemetry to communicatewith the implanted neurostimulator 20 to enable the physician to programand manage a patient's therapy and troubleshoot the system. A typicalphysician's programmer is available from Medtronic, Inc. and bears ModelNo. 7432. Similarly, a patient's programmer (also not shown) also usestelemetry to communicate with neurostimulator 20 so as to enable thepatient to manage some aspects of their own therapy as defined by thephysician. An example of a patient programmer is Model 7434 Itrel® 3 EZPatient Programmer available from Medtronic, Inc.

Implantation of a neurostimulator typically begins with the implantationof at least one stimulation lead while the patient is under a localanesthetic. While there are many spinal cord lead designs utilized witha number of different implantation techniques, the largest distinctionbetween leads revolves around how they are implanted. For example,surgical leads have been shown to be highly effective, but require alaminectomy for implantation. Percutaneous leads can be introducedthrough a needle, a much easier procedure. To simplify the followingexplanation, discussion will focus on percutaneous lead designs,although it will be understood by those skilled in the art that theinventive aspects are equally applicable to surgical leads. After thelead is implanted and positioned, the lead's distal end is typicallyanchored to minimize movement of the lead after implantation. The lead'sproximal end is typically configured to connect to a lead extension 22.The proximal end of the lead extension is then connected to theneurostimulator 20.

FIG. 2 illustrates a DBS system implanted in a patient 40 and comprisessubstantially the same components as does an SCS; that is, at least oneneurostimulator, at least one extension, and at least one stimulationlead containing one or more electrodes. As can be seen, eachneurostimulator 42 is implanted in the pectoral region of patient 40.Extensions 44 are deployed up through the patient's neck, and leads 46are implanted in the patient's brain as is shown at 48. As can be seen,each of leads 46 is connected to its respective extension 44 just abovethe ear of both sides of patient 40.

FIG. 3 is an isometric view of the distal end of lead 46. In this case,four ring electrodes 48 are positioned on the distal end of lead 46 andcoupled to internal conductors or filers (not shown) contained withinlead 46. Again, while four ring electrodes are shown in FIG. 3, it is tobe understood that the number of electrodes can vary to suit aparticular application.

FIG. 4 is an isometric view of the distal end of extension 44, whichincludes a connector portion 45 having four internal contacts 47. Theproximal end of the DBS lead, shown in FIG. 3, plugs into distalconnector 45 of extension 44 and is held in place by means of, forexample, a plurality (e.g. four) of set screws 50. For example,referring to FIG. 5, lead 46 terminates in a series of proximalelectrical ring contacts 48 (only one of which is shown in FIG. 5). Lead46 may be inserted through an axially aligned series of openings 52(again only one shown) in screw block 54. With lead 46 so inserted, aseries of set screws 50 (only one shown) are screwed into blocks 54 todrive contacts 48 against blocks 54 and secure and electrically couplelead 46. It should be appreciated, however, that other suitable methodsfor securing lead 46 to extension 44 may be employed. The proximalportion of extension 44 is secured to neurostimulator 42 as is shown inFIGS. 1 and 2.

FIG. 6 is a top view of lead 46 shown in FIG. 2. FIGS. 7 and 8 arecross-sectional views taken along lines 7-7 and 8-8 in FIG. 6. Distalend 60 of lead 46 includes at least one electrode 62 (four are shown).As stated previously, up to eight electrodes may be utilized. Each ofelectrodes 62 is preferably constructed as is shown in FIG. 8. That is,electrode 62 may comprise a conductive ring 71 on the outer surface ofthe elongate tubing making up distal shaft 60. Each electrode 62 iselectrically coupled to a longitudinal wire 66 (shown in FIGS. 7 and 8)which extends to a contact 64 at the proximal end of lead 46.Longitudinal wires 66 may be of a variety of configurations; e.g.discreet wires, printed circuit conductors, etc. From the arrangementshown in FIG. 6, it should be clear that four conductors or filers runthrough the body of lead 46 to electrically connect the proximalelectrodes 64 to the distal electrodes 66. As will be further discussedbelow, the longitudinal conductors 66 may be spirally configured alongthe axis of lead 46 until they reach the connector contacts.

The shaft of lead 46 preferably has a lumen 68 extending therethroughfor receiving a stylet that adds a measure of rigidity duringinstallation of the lead. The shaft preferably comprises a comparativelystiffer inner tubing member 70 (e.g. a polyamine, polyamide, highdensity polyethylene, polypropylene, polycarbonate or the like).Polyamide polymers are preferred. The shaft preferably includes acomparatively softer outer tubing member 72; e.g. silicon or othersuitable elastomeric polymer. The conductive rings 71 are preferably ofa biocompatible metal such as one selected from the noble group ofmetals, preferably palladium, platinum or gold and their alloys.

FIG. 9 illustrates an alternative lead 74 wherein distal end 76 isbroader or paddle-shaped to support a plurality of distal electrodes 78.A lead of this type is shown in FIG. 1. As was the case with the leadshown in FIGS. 6, 7 and 8, distal electrodes 78 are coupled to contacts64 each respectively by means of an internal conductor or filer. A moredetailed description of the leads shown in the FIGS. 6 and 9 may befound in U.S. Pat. No. 6,529,774 issued Mar. 4, 2003 and entitled“Extradural Leads, Neurostimulator Assemblies, and Processes of UsingThem for Somatosensory and Brain Stimulation”.

Leads of the type described above may be of the wound helix filer typeor of the cabled filer type. FIGS. 10 and 11 are longitudinal and radialcross-sectional views of a helically wound lead of the type shown inFIG. 6. The lead comprises an outer lead body 80; a plurality ofhelically wound, co-radial lead filers 82; and a stylet lumen 84. Asstated previously, a stylet is a stiff, formable insert placed in thelead during implant so as to enable the physician to steer the lead toan appropriate location. FIG. 10 illustrates four separate, co-radiallywound filers 86, 88, 90 and 92 which are electrically insulated fromeach other and electrically couple a single electrode 62 (FIG. 6) to asingle contact 64 (FIG. 6).

As can be seen, the lead filers 82 have a specific pitch and form ahelix of a specific diameter. The helix diameter is relevant indetermining the inductance of the lead. These filers themselves alsohave a specific diameter and are made of a specific material. The filerdiameter, material, pitch and helix diameter are relevant in determiningthe impedance of the lead. In the case of a helically wound lead, theinductance contributes to a frequency dependent impedance. FIGS. 12 and13 are longitudinal and radially cross-sectional views, respectively, ofa cabled lead. The lead comprises outer lead body 94, stylet lumen 96,and a plurality (e.g. four-to-eight) of straight lead filers 98.

FIG. 14 is an exploded view of a neurostimulation system that includesan extension 100 configured to be coupled between a neurostimulator 102and lead 104. The proximal portion of extension 100 comprises aconnector 106 configured to be received or plugged into connector block109 of neurostimulator 102. The distal end of extension 100 likewisecomprises a connector 110 including internal contacts 111 configured toreceive the proximal end of lead 104 having contacts 112 thereon. Thedistal end of lead 104 includes distal electrodes 114.

FIG. 15 is a cross-sectional view of extension 100. Lead extension 100has a typical diameter of 0.1 inch, which is significantly larger thanthat of lead 104 so as to make extension 100 more durable than lead 104.Extension 100 differs from lead 104 also in that each filer 106 in leadbody 100 is helically wound or coiled in its own lumen 108 and notco-radially wound with the rest of the filers as was the case in lead104.

The diameter of typical percutaneous leads is approximately 0.05 inch.This diameter is based upon the diameter of the needle utilized in thesurgical procedure to deploy the lead and upon other clinical anatomicalrequirements. The length of such percutaneous SCS leads is based uponother clinical anatomical requirements. The length of such percutaneousSCS leads is typically 28 centimeters; however, other lengths areutilized to meet particular needs of specific patients and toaccommodate special implant locations.

Lead length is an important factor in determining the suitability ofusing the lead in an MRI environment. For example, the greater length ofthe lead, the larger the effect of loop area that is impacted by theelectromagnetic field (i.e. the longer the lead, the larger theantenna). Furthermore, depending on the lead length, there can bestanding wave effects that create areas of high current along the leadbody. This can be problematic if the areas of high current are near thedistal electrodes.

Compared to the helically wound lead, the cable lead has a smaller DCresistance because the length of the straight filer is less than that ofa coiled filer and the impedance at frequency is reduced because theinductance has been significantly reduced. It has been determined thatthe newer cabled filer designs tend to be more problematic in an MRIenvironment than do the wound helix filer designs. It should be notedthat straight filers for cable leads sometimes comprise braided strandedwire that includes a number of smaller strands woven to make up eachfiler. This being the case, the number of strands could be varied toalter the impedance.

It has been discovered that high lead impedances at MRI operationalfrequencies can reduce the heating of an electrode during an MRIprocedure. The high impedance acts as a choke for current flowingthrough the lead and, by restricting this current, electrode heating canbe reduced. As previously alluded to, leads have been intentionallydesigned with low impedance to enhance system stimulation efficiency.The simplest way to increase the impedance of a lead is to increase itsDC resistance. This may be accomplished in a number of ways that may, ifdesired, be combined to achieve an optimal impedance.

For example, the resistance R of a lead filer is governed by theequation:

$\begin{matrix}{R = \frac{L}{\sigma \; a}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where R is the resistance, L is the length of the filer, a is theconductivity, and a is the cross-sectional area. Decreasing theconductivity and/or the cross-sectional area of the filer will increaseresistance proportionally. One typical lead utilizes a stainless steel(non-cored MP35N) filer having a conductivity of 1.1×10⁶ mhos/meter, adiameter of approximately 0.005 inch, and a length of approximately 100centimeters. Using Equation (1), the resistance R of the lead isapproximately seventy-two ohms. If the diameter were reduced to 0.002inch, R could be increased to approximately 448 ohms (or approximately126 ohms for a 28 centimeter lead). This is still not sufficient for usein an MRI-safe neurological lead.

Impedance can also be obtained through inductance in accordance with theequation:

Z=j(2πf)L  Equation (2)

where Z is the impedance, L is the inductance, and f is the frequency.Inductance L may be either distributed or discrete. For example,distributed inductance can be created by helically coiling the leadfilers in such a way as to achieve the above described optimal impedanceat MR frequencies. The inductance is governed by the equation:

$\begin{matrix}{L = \frac{\mu \; N^{2}A}{l}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

where N is the number of turns in the helix, A is the cross-sectionalarea, l is the length, and μ is the permeability.

Example 1

In the case of a 28 centimeter wound filer lead having a diameter of0.05 inch and forty turns per inch (i.e. approximately 440 turns for theentire lead), Equation (3) yields an inductance of approximately 1.1 μH.Substituting this value of inductance into Equation (2) yields anabsolute impedance of 442 ohms.

Example 2

The DC resistance of a 0.005 inch diameter MP35N straight 28 centimetersfiler is approximately 20 ohms. If it is assumed that only inductancecan be varied to achieve an impedance in the range of 375 to 2000 ohmsat 64 MHz, the inductance, as determined by Equation 2, should beapproximately 1.0 μH to 5 μH. At 128 MHz, this range of inductanceyields an impedance of approximately 804 to 4000 ohms. For a preferredimpedance of 600 ohms, the inductance is approximately 1.5 μH.

Example 3

Optimal impedance at MR frequencies is best obtained through acombination of impedance and inductance. Assume a helically coiled(approximately fifty turns per inch), 28 centimeter long lead having abraided stranded wire having a diameter of 0.002 inch. Equation 3 tellsus that the inductance is approximately 1.72 μH. Substitution intoEquation 2 yields an impedance of j691 ohms. The DC resistance of such alead is approximately 1110 ohms. Therefore, Z=1110+j691 ohms. Thus,

|Z|=(1110²+691²)^(1/2) or 1307 ohms

One known helically wound lead has a DC resistance of approximately 3.3ohms/centimeter-of-lead. Thus, the DC resistance of 28 centimeter leadis approximately 92.4 ohms, and that of a 100 centimeter lead isapproximately 330 ohms. The inductance of this known lead isapproximately 31 nH/centimeter-of-lead. Given this information andutilizing Equations (1) and (2), the impedance of known leads isapproximately 9.0 ohms/centimeter-of-lead at 43 MHz, 12.9ohms/centimeter-of-lead at 64 MHz, and 25.1 ohms/centimeter-of-lead at128 MHz. Thus, a 100 centimeter lead will have an absolute impedance ofapproximately 900 ohms at 43 MHz, 1290 ohms at 64 MHz, and 2510 ohms at128 MHz.

In view of the above, the inventive lead should have a DC resistance ofat least 375 ohms (preferably approximately 600 ohms) or alternativelyat least 5 ohms/centimeter. By utilizing high resistance wires (in theorder of 1×10⁴ mhos/centimeter) and assuming a lead length of 100centimeters, Equation (1) yields a DC resistance of approximately 2000ohms.

A discrete inductor in the form of, for example, a hybrid component orwound helix in the conductor path of the lead may be utilized to provideinductance, and therefore impedance. In this way, a frequency-dependentimpedance can be added at one of more locations in the lead. One suchlocation may be within the lead's distal electrode, which will protectthe connections to the inductor. One terminal of the inductor could beattached directly to the electrode, and the other terminal may beattached to the filer. It has been found that placing the inductor closeto the distal electrode minimizes heating during an MR scan.

The discrete inductor comprises a coil of wire of, for example,cylindrical or torroidal construction. While both may be accommodated ina cylindrical package that may fit easily inside a lead electrode, itshould be clear that packages of other shapes may be accommodated. Forexample, FIG. 16 illustrates a cylindrically packaged discrete inductor116 configured within distal electrode 114. FIG. 17 is a cross-sectionalview of a prismatically packaged discrete inductor 118 configured withindistal electrode 114, and FIG. 18 is a cross-sectional view of aquadripolar coaxially wound lead including outer lead body 120, styletlumen 122, and at least four helically and coaxially wound lead filers124. The lead shown in FIG. 18 provides for a high helix angle and leadinductances that reach or exceed 40 μH/cm. Each filer 124 can beindividually insulated or positioned in its own sleeve. Furthermore,each filer 124 may be wound in either direction; and for added strength,certain ones of the filers may be wound in opposite directions.

There are a number of techniques that may be utilized to attach adiscrete inductor to a lead electrode; e.g. welding, soldering, using aconductive epoxy, etc. Furthermore, while the discrete inductor ispreferably placed inside the distal electrodes, it could also be placedinside the proximal electrodes and still have the benefit of themechanical protection afforded by the electrode. The discrete inductorcould also be placed within the lead body.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. For example, whilethe invention has been described in connection with neurostimulationsystems, the invention is equally applicable to other lead assemblies(e.g. implantable cardiac leads) that may be adversely impacted in highfrequency environments such as is encountered during an MRI scan. Itshould also be appreciated that the exemplary embodiment or exemplaryembodiments are only examples, and are not intended to limit the scope,applicability, or configuration of the invention in any way. Rather, theforegoing detailed description will provide those skilled in the artwith a convenient road map for implementing an exemplary embodiment ofthe invention, it being understood that various changes may be made inthe function and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A medical system, comprising: a stimulator forgenerating electrical stimulation therapy; and a lead coupled to thestimulator, the lead comprising: an implantable elongate insulatingbody; an electrode on a distal end of the insulating body; and anelectrical conductor within the insulating body that is connected to theelectrode, the conductor having a calculated absolute impedance greaterthan 15 ohms per centimeter at 64 MHz.
 2. The medical system of claim 1,further comprising a contact on a proximal end of the insulating body,wherein the conductor is connected to the contact.
 3. The medical systemof claim 1, wherein the conductor includes a discrete inductor.
 4. Themedical system of claim 3, wherein the discrete inductor is locatedwithin the insulating body in proximity to the electrode.
 5. The medicalsystem of claim 1, wherein at least a portion of the conductor ishelical.
 6. The medical system of claim 1, further comprising a secondelectrical conductor within the insulating body, the electricalconductor being co-radially wound with the second electrical conductor.7. The medical system of claim 1, wherein the conductor is a helicalshape and wherein the calculated absolute impedance (|Z|) is calculatedbased on the equation:|Z|=(R ²+2πfL ₂ ²)^(1/2), where f is the frequency of 64 MHz, where R isan electrical resistance of the conductor that is calculated based onthe equation: ${R = \frac{L_{1}}{\sigma \; a}},$ where L₂ is anelectrical inductance of the conductor that is calculated based on theequation: ${L_{2} = \frac{\mu \; N^{2}A}{l}},$ where L₁ is a lengthof the conductor, where σ is a conductivity for the conductor, where ais a cross-sectional area of the conductor, where μ is a permeativityfor the space occupied by the helical shape of the conductor, where N isa number of turns of the helical shape of the conductor, where A is across-sectional area of the helical shape of the conductor, and where lis a length of the helical shape of the conductor.
 8. A medical systemcomprising: a stimulator for generating electrical stimulation therapy;and a lead coupled to the stimulator, the lead comprising: animplantable elongate insulating body; an electrode on a distal end ofthe insulating body; an electrical conductor within the insulating bodythat is connected to the electrode, the conductor having an absoluteimpedance greater than 12.9 ohms per centimeter at 64 MHz, the conductorhaving discrete elements positioned at multiple locations along theconductor where each of the discrete elements contributes to theabsolute impedance.
 9. The medical system of claim 8, further comprisinga contact on a proximal end of the insulating body, wherein theconductor is connected to the contact.
 10. The medical system of claim8, wherein at least a portion of the conductor is helical.
 11. Themedical system of claim 10, wherein a calculated absolute impedance(|Z|) of at least the portion of the conductor that is helical iscalculated based on the equation:|Z|=(R ²+2πfL ₂ ²)^(1/2), where f is the frequency of 64 MHz, where R isan electrical resistance of the portion of the conductor and iscalculated based on the equation: ${R = \frac{L_{1}}{\sigma \; a}},$where L₂ is an electrical inductance of the portion of the conductor andis calculated based on the equation:${L_{2} = \frac{\mu \; N^{2}A}{l}},$ where L₁ is a length of theportion of the conductor, where σ is a conductivity for the portion ofthe conductor, where σ is a cross-sectional area of the portion of theconductor, where μ is a permeativity for the space occupied by thehelical shape of the portion of the conductor, where N is a number ofturns of the helical shape of the portion of the conductor, where A is across-sectional area of the helical shape of the portion of theconductor, and where l is a length of the helical shape of the portionof the conductor.
 12. The medical system of claim 8, further comprisinga second electrical conductor within the insulating body, the electricalconductor being co-radially wound with the second electrical conductor.13. A medical system comprising: a stimulator for generating electricalstimulation therapy; and a lead coupled to the stimulator, the leadcomprising: an implantable elongate insulating body; an electrode on adistal end of the insulating body; an electrical conductor within theinsulating body that is connected to the electrode, the conductor havingan absolute impedance greater than 12.9 ohms per centimeter at 64 MHzwhile having a DC resistance less than 12.9 ohms per centimeter.
 14. Themedical system of claim 13, further comprising a contact on a proximalend of the insulating body, wherein the conductor is connected to thecontact.
 15. The medical system of claim 13, wherein the conductorincludes a discrete inductor.
 16. The medical system of claim 15,wherein the discrete inductor is located within the insulating body inproximity to the electrode.
 17. The medical system of claim 13, whereinat least a portion of the conductor is helical.
 18. The medical systemof claim 13, further comprising a second electrical conductor within theinsulating body, the electrical conductor being co-radially wound withthe second electrical conductor.
 19. The medical system of claim 13,wherein the conductor is a helical shape and wherein the calculatedabsolute impedance (|Z|) is calculated based on the equation:|Z|=(R ²+2πfL ₂ ²)^(1/2), where f is the frequency of 64 MHz, where R isan electrical resistance of the conductor that is calculated based onthe equation: ${R = \frac{L_{1}}{\sigma \; a}},$ where L₂ is anelectrical inductance of the conductor that is calculated based on theequation: ${L_{2} = \frac{\mu \; N^{2}A}{l}},$ where L₁ is a lengthof the conductor, where σ is a conductivity for the conductor, where ais a cross-sectional area of the conductor, where μ is a permeativityfor the space occupied by the helical shape of the conductor, where N isa number of turns of the helical shape of the conductor, where A is across-sectional area of the helical shape of the conductor, and where lis a length of the helical shape of the conductor.
 20. A medical systemcomprising: a stimulator for generating electrical stimulation therapy;and a lead coupled to the stimulator, the lead comprising: animplantable elongate insulating body; a conductive body on a distal endof the insulating body; an electrical conductor within the insulatingbody, the conductor having an absolute impedance greater than 12.9 ohmsper centimeter at 64 MHz, the conductor having an inductor that isdisposed within the conductive body.
 21. The medical system of claim 20,wherein the conductor is connected to the conducting body.
 22. Themedical system of claim 20, wherein at least a portion of the conductoris helical.
 23. The medical system of claim 22, wherein a calculatedabsolute impedance (|Z|) of at least the portion of the conductor thatis helical is calculated based on the equation:|Z|=(R ²+2πfL ₂ ²)^(1/2), where f is the frequency of 64 MHz, where R isan electrical resistance of the portion of the conductor and iscalculated based on the equation: ${R = \frac{L_{1}}{\sigma \; a}},$where L₂ is an electrical inductance of the portion of the conductor andis calculated based on the equation:${L_{2} = \frac{\mu \; N^{2}A}{l}},$ where L₁ is a length of theportion of the conductor, where σ is a conductivity for the portion ofthe conductor, where σ is a cross-sectional area of the portion of theconductor, where μ is a permeativity for the space occupied by thehelical shape of the portion of the conductor, where N is a number ofturns of the helical shape of the portion of the conductor, where A is across-sectional area of the helical shape of the portion of theconductor, and where l is a length of the helical shape of the portionof the conductor.
 24. The medical system of claim 20, further comprisinga second electrical conductor within the insulating body, the electricalconductor being co-radially wound with the second electrical conductor.